[InstCombine] canonicalize fneg after shuffle

For the unary shuffle pattern, this is opposite to what we try
to do with binops, but it seems better to keep it consistent
with the motivating binary shuffle pattern. On that, it is
clearly better on the usual no-extra uses case.

There is a chance that this will pull an fneg away from some
other binop and cause a regression in codegen, but that should
be invertible in the backend. The transform is birectional:
https://alive2.llvm.org/ce/z/kKaKCU
https://alive2.llvm.org/ce/z/3Desfw

Fixes #45631